Salem

http://salemwiki.info/index.php/Clay_Trough

Can you fill the trough with water 1st and then add lime directly to the trough afterwards... in the same way as brains into a bucket 1st or directly into the tanning tub if you wish?

And what about this:?
search.php?keywords=clay+trough&terms=all&author=&sc=1&sf=titleonly&sr=posts&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

And is there a way to see how many litres Milk of Lime you have remaining in the trough?

Im pretty sure its no and no, keep a bucket or two of lime water in your inventory when making clay

You CAN save lime by diluting your Milk of Lime in a barrel, but too much dilution can make it unusable. 1 Lime for 3 buckets of water is safe.

2 lime in a full barrel of water should be fine if it's not fixed.

Good to know that Milk of lime is good down to 0.2% (or 1:5 dilution). Hadn't experimented enough, but I knew 0.1 was no good.

Tanning fluid however has sterner restrictions on concentration. 5% and you're good to go, 4.9% and NOTHING. Graphically it's amusing to drop 0.1 liters of water into a basin and watch the fluid disappear.